1. Field of the Invention
This invention relates to minimizing NOx in exhaust gasses produced by internal combustion engines and more particularly relates to determining NOx levels produced by individual combustion chambers and modifying selected engine parameters in response to NOx levels.
2. Description of the Related Art
When combustion occurs in an environment with excess oxygen, peak combustion temperatures increase, leading to the formation of unwanted emissions, such as oxides of Nitrogen (NOx). This problem is often aggravated in the presence of turbocharging, which increases the mass of fresh air flow, thereby increasing the mass of oxygen and nitrogen present in a combustion chamber.
One known technique for reducing unwanted emissions such as NOx involves introducing chemically inert gases into the fresh air stream for subsequent combustion. By reducing the oxygen concentration of the fresh air stream, the fuel burns slower and peak combustion temperatures are reduced, thereby lowering the production of NOx. In an internal combustion engine environment, such chemically inert gases are readily abundant in the form of exhaust gases, and one known method for introducing inert gases is through the use of an Exhaust Gas Recirculation (EGR) system. EGR systems introduce exhaust gas from the exhaust manifold into the fresh air stream flowing to the intake manifold valve. However, lowering combustion temperatures in such a manner may actually increase emissions of particulate matters and unburned hydrocarbons.
Other methods for reducing NOx include catalytic converters, and ignition/injector timing control. Catalytic converters typically use two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both the reduction catalyst and the oxidization catalyst are commonly embedded on the surface of a ceramic honeycomb-like structure in order to achieve a maximum surface area of catalyst in the exhaust stream. In both cases, a metal, such as platinum, rhodium, and/or palladium, serves as the catalyst.
The further component of the catalytic converter is the engine control module (ECM) that monitors the exhaust stream, and controls a fuel injection system in response to detected NOx levels. One method for monitoring the exhaust stream is an oxygen sensor typically installed upstream from the catalytic converter. The oxygen sensor is configured to indicate how much oxygen is in the exhaust. The ECM may subsequently increase or decrease the amount of air being injected into the combustion chamber. Alternatively, the oxygen sensor may be installed downstream from the catalytic converter in order to detect NOx not being converted by the catalytic converter.
Currently, dedicated NOx sensors are mostly solid-state electrochemical sensors. Yttria-stabilized zirconia (YSZ) sensors have been used in the exhaust flow to detect NOx at high temperatures. However, poor accuracy and slow response times plague YSZ sensors and thereby limit their usability. Furthermore, since the sensitivity of an YSZ sensor is also affected by changes in gas composition, in particular the oxygen concentration, cross sensitivity is a major interference problem.
However, a major drawback of common NOx sensors is the inability to identify correctly which combustion chamber is producing NOx. NOx sensors are commonly located downstream of the exhaust manifold. As used herein, downstream refers to a direction in which exhaust gas travels away from the engine. FIG. 1 is a perspective side view diagram depicting a portion of an internal combustion engine 100 (hereinafter “engine”) having an exhaust manifold in accordance with the prior art. The engine 100 typically includes a cylinder block 102 coupled to a cylinder head 104.
The cylinder head 104 has intake ports (not shown) for the intake of combustive material, such as an air-fuel mixture, and exhaust ports (not shown) for the exhaust of the combustive material. The exhaust ports are generally coupled to an exhaust manifold 106 that collects exhaust gasses from each combustion chamber or cylinder. A turbocharger 108 may be coupled to the exhaust manifold.
FIG. 2 is a schematic block diagram illustrating a top view of the cylinder head 104 and the exhaust manifold 106 in accordance with the prior art. Many underlying engine components, which are well known to those skilled in the art, but which may not be relevant to this discussion, have been omitted and will not be discussed in great detail herein. The flow of exhaust gasses flows in a direction as indicated by arrow 202, away from the cylinder head 104 and towards a catalytic converter 204. The catalytic converter 204, as described above, reduces NOx in the exhaust gas flow. An oxygen sensor 206 adapted to detect NOx may be positioned downstream from the catalytic converter 204 in order to detect NOx that passes through the catalytic converter 204.
Feedback from the oxygen sensor 206 is typically transmitted to an engine control module 208. The engine control module 208 may then modify engine parameters in order to reduce NOx production inside the cylinders. Research has indicated that proper control of air/fuel ratio can lower NOx emissions. However, current NOx sensors are unable to detect which cylinder is producing the NOx, and the engine control module 208 must therefore modify timing and or air/fuel ratio of all cylinders. This often results in increased particulate emissions, as described above, and reduced engine performance.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that detect and correct for NOx in exhaust gasses. Such an apparatus, system, and method would be further beneficial if they were capable of detecting the levels of NOx produced in individual cylinders, and selectively modifying engine parameters in response to the detected NOx levels.